The present invention relates to a method for burning nitrogen-containing fuels and apparatus therefor.
Air pollution has become a major problem in the United States and other highly industrialized countries of the world. Consequently, the control and/or reduction of said pollution has become the true object of major research and development efforts by both governmental and nongovernmental agencies. It has been alleged, and there is supporting evidence, that automobiles employing conventional piston-type engines which burn hydrocarbon fuels are a major contributor to said pollution. Vehicle emission standards have been set by the United States Environmental Protection Agency (EPA) which are sufficiently restrictive to cause automobile manufacturers to consider employing alternate engines instead of the conventional piston engine.
Another source of such pollution is the exhaust gases from large stationary installations such as boilers in power plants, and large stationary gas turbine engines employed as a driving force in power plants and other large installations. It is anticipated that this portion of the problem will almost certainly be aggravated in the relatively near future by the necessity to use lower quality fuels such as heavy petroleum oils, shale oils, coal liquids, etc., which contain relatively large amounts of fuel-nitrogen, e.g., up to about 2 weight percent or greater, as compared to presently available fuels which contain very little, if any, fuel-nitrogen. For example, #4 and #6 petroleum oils contain about 0.1 to 0.5 weight percent nitrogen, two typical crude shale oils, hereinafter referred to in the specific examples, contain 1.85 and 1.93 weight percent of chemically bound nitrogen, respectively, and a typical crude, solvent-refined coal oil contains from about 1.0 to 1.5 weight percent of chemically bound nitrogen. By comparison, a typical petroleum-derived #2 fuel oil contains about 0.024 weight percent nitrogen. If all of the chemically bound nitrogen in the fuel, generally referred to as "fuel NO.sub.x ", is converted to nitrogen oxides, 1 percent by weight of nitrogen in a solvent-refined coal oil, has the potential to produce about 1.928 pounds/million Btu or 1,300 ppmv (parts per million by volume at 3 percent excess oxygen, dry) of nitrogen oxides (NO.sub.x) while 1.85 and 1.93 percent by weight of nitrogen in crude shale oils will potentially produce about 3.288 and 3.440 pounds/million Btu (2595 and 2642 ppmv), of NO.sub.x respectively. In addition, nitrogen oxides, produced by the hot-air reactions at flame temperatures, and referred to as "thermal NO.sub.x ", also contribute to the total NO.sub.x pollutants in the flue gases from a combustion process.
The federal limit for the discharge of NO.sub.x pollutants into the atmosphere from steam generators burning liquid fossil fuel (1974 EPA New Source Performance Standards [NSPS]) is 0.30 pounds/million Btu (about 230 to 37 ppmv for typical shale oils). Some state limitations are even more stringent, for example the California standard is 225 ppmv. These limits include both fuel NO.sub.x and thermal NO.sub.x. While these standards can be met when burning low nitrogen (below about 0.1%), petroleum-derived fuel oils, serious complications are encountered when high nitrogen fuels such as heavy petroleum-derived fuel oils, crude shale oils and crude coal oils are burned under conventional utility boilers. For example, since thermal NO.sub.x increases with temperature, modern utility boiliers, which preheat the combustion-supporting air to 600.degree.-800.degree. F. for improved efficiency, produce thermal NO.sub.x alone which can approach the specified emission standards. Consequently, in order to meet these standards, the conversion of fuel nitrogen to NO.sub.x emissions, in a fuel having about 2.0 weight percent bound nitrogen, should not be more than about 5 percent. It has been reported in the literature that, when shale oils with about 2.0 weight percent nitrogen are burned in a stationary boiler of an electrical generating station, NO.sub.x emissions on the order of 700 to 900 ppmv can be anticipated and, when solvent refined coal oils, with slightly more than 1.0 weight percent nitrogen, are burned, at least 20 to 50 percent of the fuel nitrogen is converted to NO.sub.x emissions (260 to 650 ppmv).
While it has been suggested that high levels of fuel nitrogen can be reduced by severe hydro-treating, such techniques have not been commercially developed and, even if available, pilot plant tests indicate that such refining of crude shale oils and crude coal oils would increase costs by about $3.00 to $5.00 per barrel.
It has also been suggested that NO.sub.x emissions from crude, high nitrogen fuels can be reduced by blending the high nitrogen fuel with low nitrogen petroleum-derived fuel oils or by burning crude, high nitrogen fuels in selected burners of a boiler while burning low nitrogen petroleum-derived fuel oils in other burners. In addition to requiring substantial volumes of petroleum-derived fuel oils, these techniques require additional equipment for handling and blending and/or feeding two separate fuels.
Another suggestion for reducing NO.sub.x emissions from high nitrogen fuels is the addition of a fuel additive, such as an additive containing manganese. Obviously, this technique adds the cost of the additive to the operation and requires facilities for handling and blending the additive.
To date the most promising technique has been a two-stage, rich-lean combustion process, in which a primary combustion zone is operated fuel-rich and a secondary combustion region is operated fuel-lean. However, even with the best of these techniques only limited success has been attained and the conversion of fuel-nitrogen to NO.sub.x emissions is still well above the governmental limits. For example, it has been reported that the NO.sub.x level, when burning crude, solvent refined coal oil containing 1.12 weight percent nitrogen, cannot be reduced below about 0.4 lb./million Btu without resorting to the additional techniques of blending with low nitrogen, petroleum-derived fuels or using additives. Similarly, the best of the two-stage techniques, heretofore available, have failed to reduce NO.sub.x emissions from crude shale oils, containing from 2.0 weight percent nitrogen, to values less than about twice the governmental maximums.
The major problems in past operations of two-stage, rich-lean combustion processes has been the failure to recognize that control of factors other than the rich fuel-air ratio is necessary in order to attain minimum NO.sub.x production, and that the efficiency of fuel utilization and control of other pollutants must also be taken into consideration. The almost universal thinking of those skilled in the art has been that, so long as combustion is initiated in a fuel-air mixture having a fuel-air ratio above the stoichiometric ratio (usually referred to as a fuel-air equivalence ratio above 1.0) and the flame is thereafter diluted with air to reduce the overall fuel-air ratio to below the stoichiometric ratio (a fuel-air equivalence ratio below 1.0), this is all that is necessary in order to accomplish the desired result. The ultimate efficiency of fuel utilization and the volume of unburned or partially burned fuel (generally indicated by the HC and CO content of the flue gases) has also been generally ignored by such prior art investigators.